During the preview period you needed to provide the application/vnd.github.polaris-preview preview media type in the Accept header to opt-in to the changes. Now that the preview period has ended, you no longer need to specify this custom media type.

Note: During the preview period, we initially added a squash property in the payload to the pull request merge endpoint. This property was
superseded by a merge_method property to support both "squash and merge" as well as "rebase and merge" operations.
With the end of the preview period, support for the squash property was removed in favor of the merge_method property.